1. Field of the Invention
This invention relates to improvements in an airbag of a vehicular airbag restraint system, and more particularly to the airbag formed of a doubly woven cloth defining therein a gas chamber to be filled with high pressure gas from an inflator.
2. Description of the Prior Art
There are a variety of airbag restraint systems which have been proposed and put into practical use. These systems are usually constructed and arranged as follows: When a vehicle deceleration, due to a vehicle collision, exceeds predetermined level, an initiator is ignited to start the operation of an inflator or gas generator, thereby inflating and developing an airbag from its folded state. Accordingly, a vehicle passenger is protected from coming into direct contact with an instrument panel and/or a front windshield of an automotive vehicle.
The airbag is usually formed of front and rear side singly woven cloths, in which the woven cloths are sewn at their peripheral portions with each other to form a bag-shape. However, frays tends to occur at the peripheral portions of the singly woven cloths, while the singly woven cloths are insufficient in strength. For such an airbag, it is necessary to separately provide to the peripheral portion a sewn part for the purpose of reinforcement, thereby increasing the number of steps in the production process of it and the production cost of the same.
In view of the above, the airbag has been recently proposed to form an airbag of a doubly woven cloth in place of the above-mentioned singly woven cloths. An example of such an airbag formed of the doubly woven cloth is disclosed in Japanese Patent Provisional Publication No. 2-158442. Such an airbag includes front an rear side (singly woven) cloth parts which are separate from each other, but which are integral at their peripheral portion with each other thereby defining therebetween a gas chamber to be fed with high pressure gas from an inflator.
In the production of such an airbag, it is unnecessary to sew the peripheral portions of two separate (singly woven) cloths with each other to form the airbag. Additionally, it is also unnecessary to reinforce the peripheral portions of the cloths. Thus, a sewing step can be omitted, thereby lowering the production cost while simplifying a production process of the airbag.
However, drawbacks have been encountered in the airbag formed of the doubly woven cloth, in which when a stress is concentrically and locally applied to the airbag from the outside, the airbag tends to be broken under the influence of such a stress. More specifically, the airbag usually requires a central opening (not shown) in one of the cloth parts. Through the central opening, high pressure gas is supplied inside the airbag. Accordingly, the airbag tends to easily tear or break when a high stress is concentrically and locally applied to the airbag cloth at a portion around the central opening. Additionally, inflation regulating belt members may be required to be disposed inside the airbag in order to prevent the airbag from unnecessarily inflating. The belt members are usually sewn to the inner surface of the airbag. It will be understood that when a stress is concentrically and locally applied to the woven cloth of the airbag through each belt member, the airbag tends to easily tear or break at the positions where the belt members are sewn.
Thus, the airbag tends to tear or break at particular positions where the high stress is concentrically applied. When a tearing or breakage occurs in the airbag, the airbag cannot inflate uniformly to obtain a predetermined desirable shape, thereby providing a non-uniform impact softening force against a vehicle passenger during a vehicle collision or the sudden stopping of a vehicle. In order to prevent such drawbacks, it is necessary to provide a reinforcement patch around the central opening of the airbag and reinforcement members at positions where the inflation regulating belt members are sewn to the airbag. These reinforcement members unavoidably complicate the production process and raise the production cost of the airbag.